1. Field of the Invention
The present invention relates to an optical coherence reduction method and device, an illuminating method and system and an optical fiber bundle.
2. Description of the Related Art
Heretofore, for a light source for illumination used for a luminaire for a projection-type liquid crystal display, measuring device and others, incoherent light source such as a lamp and a light emitting diode (LED) have been used because of various reasons such as costs and simplicity.
A laser beam emitted from a laser such as a solid state laser, a gas laser or a semiconductor laser has been used for illumination. A laser beam is excellent in directivity, is simultaneously provided with high luminous intensity and is a light beam high in coherence. However, a technically most difficult problem is xe2x80x9cspecklexe2x80x9d (i.e. speckle noise) caused by high coherence.
For example, a semiconductor laser is a light source, the photoelectric conversion coefficient of which is very high and which emits a laser beam excellent in directivity. However, a semiconductor laser has rarely been used for a light source for illumination because of a speckle problem caused by high coherence.
In the 1970s, research on a display using a laser beam (hereinafter called a laser display) was conducted in various places. However, one of the problems which prevented practicality was the generation of speckle in addition to problems such as the shortage of output from a light source and a modulating method.
Recently, there has been rapid progress in the technical development of elements which are key components of a laser display, such as: a high output laser using the conversion of wavelength by a solid state laser; a semiconductor laser which can oscillate beams in three primary colors of red (R), green (G) and blue (B); a spatial light modulator (a light bulb) using liquid crystal and a micro machine; and other elements.
When N pieces of speckle patterns incoherent mutually, (that is, which do not interfere with one another and are not correlated) are overlapped, the sum is equivalent to the sum of the intensity of each speckle pattern and the contrast of speckles is reduced by up to 1/xc2x7N.
Therefore, if N pieces of optical fiber are bundled and the length of each optical fiber is differentiated by a quantity in which coherence is lost, the interference between the optical fibers can be ignored. The resulting speckle is equivalent to the overlap of the intensity of speckle patterns I1, I2 . . . , IN caused by each optical fiber. Therefore, the contrast of a speckle is reduced by equalization. That is, if N pieces of speckle patterns which are not correlated and are equal in intensity are overlapped, the contrast becomes 1/xc2x7N.
A speckle (pattern) has also been a large problem in the field of a semiconductor exposure system and measures against it have been taken. The problem exists because an excimer laser has been adopted as a short wave-length light source to enhance resolution.
For example, in exposure processing related to a semiconductor device, a coherence reduction method is proposed that provides for a flying eye lens 20 composed of elements different in length, as shown in FIG. 7, used for the control of coherence. That is, a measure against speckle and a lens 21 is arranged in a position apart from flying eye lens 20 by distance f between the end face on the side of outgoing radiation of the flying eye lens 20 and a mask 22. (Refer to Lighting Optical System written by Messrs. Shibuya and Uehara and Japanese published examined patent application No. Sho 60-230629.)
However, according to this method, because the length of each element of the flying eye lens 20 is extended and the size of a lighting area from each element is different (as shown in FIG. 7), problems result, including a reduction in efficiency.
It is proposed in Japanese Patent Application No. Sho 63-22131 to realize a similar effect using a prism 23 shown in FIG. 8. However, as for the above method, the coherence reduction effect is insufficient and the optical loss is large.
In principle, a similar effect can be obtained using the dispersion of a refractive index. However, obtaining a sufficient effect by using the normal dispersion of a refractive index has been hampered by a problem in that an element becomes huge to reduce coherence.
In addition, other coherence control methods have been proposed. However, according to any of these methods, a speckle caused between an illuminating body and the naked eye cannot be sufficiently reduced in a display, a microscope, etc. Further, to remove a speckle, coherence control more severe than in a projection exposure system according to lithography and others is required.
That is, as shown in FIG. 9, the image 27 of an object 24 illuminated by an illuminating beam a, is formed on a screen 26 via a lens 25. When a phase is disturbed at random because of the rough surface of the object 24 or the state of the optical surface of the lens 25 in the case illuminating beam a is coherent light, a speckle is included in the image 27 on the screen 26.
Further, as schematically shown in FIG. 10, this means imaging the image on a screen 32 of an object 30 via a lens 31 on a retina 34 in an eye ball 33, that the image of the object on the screen via the lens may be observed with eyes. That is, in this process, the random displacement of a phase is caused on an optical path by the confusion of a beam on the screen 32 and the eye-ball 33 and a speckle is also caused in an imaging process. If spatial coherence operates on the plane of an image even if a speckle is not superposed on the image on the screen 32, a secondary speckle is caused on the retina 34 or on the retina of an observer 28 shown in FIG. 9.
The effect upon a speckle caused on the naked eye cannot be reduced by much even if these methods are used, because methods, such as the fluctuation of a mirror and a rotary diffusion plate, used in a projection exposure system based upon lithography does not reduce coherence but only moves and averages a speckle. To apply these methods to a display and others, vibration of a screen is required so that positional relationship between an illuminating body, such as a screen and an eye, is varied. (Refer to pp. 1290 to 1294 of xe2x80x9cSpeckle-free rear-projection screen using two close screens in slow relative motionxe2x80x9d, Vol. 66, No. 11 of Journal of Optical Society of America written by Epic G. Rawson, Antonio B. Nafarrate, Robert E. Norton, Joseph W. Goodman and published in November, 1976.) However, the above method is very inconvenient for practical purposes.
Heretofore, an optical fiber has been developed mainly for communication and for its material. Conventionally, a glass fiber mainly composed of quartz has been mainly used. To avoid mode dispersion, a single-mode optical fiber has been mainly developed.
As for a glass fiber, dispersion increases in a visible short wave-length region and its transmissivity is deteriorated. Therefore, the application of an optical fiber to visible light has been limited to an illuminating multimode optical fiber bundle which does not require transmission over a long distance. Particularly, because the intensity distribution of an outgoing beam is uniform if a multimode optical fiber is used, it is also a large merit that a complicated optical system such as a flying eye lens is not required.
Recently, a plastic multimode optical fiber has been developed and has attracted attention. (Refer to pp. 4261 to 4266 of xe2x80x9cGraded-index polymer optical fiber for high-speed data communicationxe2x80x9d, Vol. 33, No. 19 of Applied Optics written by Takaaki Ishigure, Eisuke Nihei and Yasuhiro Koike and published on Jul. 1, 1994.) Compared with glass fiber, plastic multimode optical fiber is low-priced, light, and shows the largest transmission efficiency in a visible region. Further, its multimode dispersion is also very large, compared with that of formal glass fiber.
Recently, a hollow waveguide for transmitting an ultraviolet laser beam has also been researched. (Refer to xe2x80x9cImprovement of a hollow waveguide for transmitting ultraviolet laser powerxe2x80x9d written by Messrs. Tsubokura, Hashishin and Kubo of the 58th Applied Physics lecture meeting manuscripts 3a-SR-18.)
It is already know that the contrast of a speckle is reduced by the multimode dispersion of coherent light transmitted in a multimode optical fiber. (Refer to pp. 128 to 134 of xe2x80x9cThe fluctuation characteristics and a speckle of an optical fiberxe2x80x9d of Vol. 8, No. 3 of Optics written by Mr. Imai and published in 1979.)
That is, as shown in FIG. 11, in a multimode optical fiber 2 composed of a core 6 and a clad 5, light beams respectively provided with a different mode component correspond to light beams incident at different times (t1, t2 and t3) at the outgoing end 38 of the multimode optical fiber 2, because a laser beam (a light beam) 36 and a laser beam (a light beam) 37 respectively provided with a different mode are respectively provided with different propagation velocities. Therefore, if a spread by the above multimode dispersion is larger than the coherent length, the coherence of an outgoing beam is deteriorated.
However, in such an independent multimode optical fiber, it is difficult to propagate a laser beam of sufficiently large luminous intensity. Also, as a laser beam outgoing from each optical fiber is provided with coherence even if such multimode optical fibers are bundled, it is difficult to control coherence, (that is, to sufficiently reduce a speckle). Moreover, to apply a multimode optical fiber to illumination for practical use, an optical fiber having large dispersion and high transmissivity in the visible area is required.
D. Gloge ignored the mode in the radial direction of a multimode weakly-coupled optical fiber, analyzed in relation to only the propagation on a time base of power and acquired the impulse response of the multimode optical fiber. (Refer to pp. 801 to 816 of Vol. 52, No. 6 of xe2x80x9cBell Syst, Tech. Jour.xe2x80x9d written by D. Gloge and published in 1973, pp. 1767 to 1783 of Vol. 51, No. 8 of xe2x80x9cBell Syst. Tech. Jour.xe2x80x9d written by D. Gloge and published in 1972 and Chapter 9 of xe2x80x9cOptical Fiberxe2x80x9d written by Messrs. Ogoshi, Okamoto and others published by Ohm in 1983.)
In other words, if the modes of light beams propagated in an optical fiber are weakly coupled, the movement of power occurs between the modes because of the fluctuation of a refractive index during propagation, the bending of the optical fibers and other factors . Particularly, if there, are multiple modes and a light beam is propagated via the movement of power between modes many times, one input beam spreads in some modes. In such a case, only the transfer function in the direction of the optical axis of an optical fiber is important. Therefore, modes having the same propagation constant based upon an optical axis can be effectively analyzed statistically as one group.
In the 1970s, it as proved that the contrast of a speckle was reduced by inserting an irregular fiber bundle. (Refer to pp. 24 to 28 of xe2x80x9cSpeckle reduction in pulsed-laser photographsxe2x80x9d of Opt. Commu., 12 written by D. Kohler, W. L. Seitz, T. R. Loree and D. Gardne and published in 1974.) Further, it is proposed to reduce a speckle using a bundle of optical fibers (i.e. an optical fiber bundle) the coherence length of which is longer than the coherence length of a laser beam source. (Refer to Japanese Published Unexamined Patent Application No. Hei 6-167640 and Japanese Patent Application No. Hei 104-5646 applied by these applicants on Feb. 6, 1998.)
However, when the number of fiber bundles is increased to reduce the contrast of a speckle, the difference in length between the shortest optical fiber and the longest optical fiber becomes rather large even if the difference in length between fibers, that is, the difference in optical path length, is slight. For example, if a semiconductor laser in a single mode is used for a light source, the typical coherence length of the light source is approximately 1 m. As a system must be large-sized to generate such a difference in optical path length, and the increase of the number of fiber bundles is difficult.
If one hundred optical fibers are bundled and each outgoing end and each incident) end are respectively aligned, even if a difference in optical path length of 1 cm is made between optical fibers, the difference in length between the shortest optical fiber and the longest optical fiber is 1 m. In this case, handling is very inconvenient.
Further, it is desirable that the length of an optical fiber (that is, the difference in optical path length in an optical fiber bundle) is made as short as possible from the viewpoint of efficiency in utilizing light because the transmissivity is deteriorated if and optical fiber is extended, as disclosed in the above Japanese published unexamined patent application No. Hei 6 -167640 and others.
It is difficult to utilize various lasers for a light source as illumination because a simple, low-priced and high-performance coherence control technique has not been disclosed in the publications described above. This prevents application to a luminaire using a laser beam, such as a display.
Particularly, in case the above problems are to be solved, e.g., using an optical fiber bundle, there arise major difficulties in that the length of an optical fiber is extended and loss during propagation in the optical fiber is increased. Also, it is inconvenient to handle optical fibers because the length of each optical fiber is greatly different, etc.
The present invention is made in view of the above situation and the object is to provide an optical coherence reduction method and a corresponding device for sufficiently reducing the coherence of a light beam using an optical fiber bundle obtained by bundling multimode optical fibers of different length so that the number of optical fibers in the bundle is a minimum.
Another object of the present invention is to provide an illuminating method and a corresponding system for utilizing a light beam obtained by reducing the coherence and speckle noise of a light beam.
Still another object of the present invention is to provide an optical fiber bundle in which multimode optical fibers of different length are bundled, so that the number of the optical fibers is the minimum required to sufficiently reduce the coherence of a light beam.
The present invention relates to an optical coherence reduction method (hereinafter, called a coherence reduction method) for reducing the coherence of incident coherent light via an optical fiber bundle in which these multimode optical fibers are bundled using plural multimode optical fibers provided with a difference in optical path length exceeding the coherence length between outgoing light beams (that is, having a difference in optical path length exceeding the coherence length between outgoing light beams).
According to the coherence reduction method of the present invention, as the coherence of incident coherent light is reduced via an optical fiber bundle obtained by bundling plural multimode optical fibers having a difference in optical path length exceeding the coherence length between an outgoing light beam outgoing from the multimode optical fiber, the difference in optical path length between two arbitrary multimode optical fibers in the above optical fiber bundle exceeds the coherence length of an outgoing light beam from either multimode optical fiber. Therefore, the minimum difference in optical path length which is necessary to sufficiently reduce the coherence of incident coherent light is generated.
Particularly, plural optical fibers having a difference in optical path length exceeding the coherence length of a light beam from a light source are not required to be bundled because an optical fiber bundle (i.e. a multimode optical fiber bundle) obtained by bundling plural multimode optical fibers having a difference in optical path length exceeding the coherence length of an outgoing light beam from the multimode optical fiber is used and the coherence length of an outgoing light beam from the multimode optical fiber is sufficiently shorter than the coherence length of incident coherent light. The[, the] length of each optical fiber (that is, the difference in optical path length) can be sufficiently reduced, compared with a case that plural optical fibers are bundled as described above. Therefore, an optical fiber bundle in which each optical fiber is bundled can be miniaturized and lightened. Simultaneously, the coherence of incident coherent light can be sufficiently reduced.
The present invention also provides an optical coherence reduction device (hereinafter, called a coherence reduction device according to the present invention) wherein plural multimode optical fibers provided with a difference in optical path length exceeding the coherence length of an outgoing light beam (that is, having a difference in optical path length exceeding the coherence length of an outgoing light beam) are used and the coherence of incident coherent light is reduced via an optical fiber bundle in which these multimode optical fibers are bundled as a device for repeatedly executing the above coherence reduction method according to the present invention.
The present invention also includes an illuminating method (hereinafter, called an illuminating method according to the present invention) in which plural multimode optical fibers provided with a difference in optical path length exceeding the coherence length of an outgoing light beam (that is, having difference in optical path length exceeding the coherence length of an outgoing light beam) are used. The coherence of incident coherent light outgoing from a light source is reduced via an optical fiber bundle in which these multimode optical fibers are bundled and a light beam, the coherence of which is reduced, is utilized for illumination.
According to the illuminating method of the present invention, the difference in optical path length between two arbitrary multimode optical fibers in the above optical fiber bundle exceeds the coherence length of an outgoing light beam from either multimode optical fiber because the coherence of incident coherent light outgoing from a light source is reduced via an optical fiber bundle in which plural multimode optical fibers having a difference in optical path length exceeding the coherence length of an outgoing light beam, outgoing from the multimode optical fiber, are bundled. Therefore, the minimum difference in optical path length which is necessary to reduce the coherence of incident coherent light sufficiently is generated and a light beam the coherence of which is reduced can be effectively utilized for illumination.
Particularly, plural optical fibers having a difference in optical path length exceeding the coherence length of a light beam from a light source are not required to be bundled because an optical fiber bundle obtained by bundling plural multimode optical fibers having a difference in optical path length exceeding the coherence length of an outgoing light beam from the multimode optical fiber is used and the coherence length of an outgoing light beam from the multimode optical fiber is sufficiently shorter than the coherence length of incident coherent light, as disclosed in the above Japanese Published Unexamined Patent Application No. Hei 6-167640 and others. The length of each optical fiber (that is, the difference in optical path length) can be sufficiently reduced, compared with a case that plural optical fibers are bundled as described above. Therefore, an optical fiber bundle in which each optical fiber is bundled can be miniaturized and lightened. Simultaneously, the coherence of incident coherent light can be sufficiently reduced and a light beam for illumination the speckle noise of which is small can be obtained.
The present invention also provides a luminaire (hereinafter, called a luminaire according to the present invention) wherein plural multimode optical fibers having a difference in optical path length exceeding the coherence length of an outgoing light beam are used. The coherence of incident coherent light outgoing from a light source is reduced via an optical fiber bundle in which these multimode optical fibers are bundled. The outgoing light beam, the coherence of which is reduced, is utilized for illumination.
Further, the present invention provides an optical fiber bundle (hereinafter, called an optical fiber bundle according to the present invention) in which plural multimode optical fibers provided with a difference in optical path length exceeding the coherence length of an outgoing light beam (that is, having a difference in optical path length exceeding the coherence length of an outgoing light beam) are bundled.
Plural optical fibers having a difference in length exceeding the coherence length of a light beam from a light source are not required to be bundled because in the optical fiber bundle according to the present invention, plural multimode optical fibers having a difference in optical path length exceeding the coherence length of a light beam outgoing from each multimode optical fiber are bundled and the coherence length of a light beam outgoing from the multimode optical fibers is sufficiently shorter than the coherence length of incident coherent light, as disclosed in the above Japanese Published Unexamined Patent Application No. Hei 6-167640 and others. The length of each optical fiber (that is, the difference in optical path length) can be sufficiently reduced, compared with the above case. Therefore, the optical fiber bundle can be miniaturized and lightened. Simultaneously an optical fiber bundle provided with the capability for sufficiently reducing coherence can be obtained.